1. Field of the Invention
The present invention relates to a single-phase induction motor in a hermetic compressor used in a refrigerator or an air conditioner.
2. Description of the Prior Art
FIG. 11 is a cross sectional view of a rotor 9 of a conventional single-phase induction motor disclosed in the Japanese Laid-open Utility Model publication No.58-172015. In FIG. 11, the rotor 9 comprises a circumference 9a of the rotor 9, slot portions 12 filled with die-cast aluminum, which constitutes a closed space in the rotor 9, teeth 13 between each slot portion 12, and bridges 14 for connecting each tooth 13 on the rotor circumference 9a side, respectively.
FIG. 12 is a cross sectional view of a rotor 9 of a conventional single-phase induction motor disclosed in the Japanese Laid-open Patent No. 60-39352. In FIG. 12, the rotor 9 comprises a circumference 9a of the rotor 9, slot opening portions 12a, and teeth 13, respectively.
In the rotor 9 in FIG. 11, the slot portion 12 constitutes a closed space in the rotor 9. The inside of the closed space is filled with the die-cast aluminum. Since the shape of the slot portion 12 is closed, it is generally referred to as a closed slot. In other words, the bridges 14 are laid between the rotor circumference 9a and the respective slot portions 12 to connect each tooth 13. The efficiency of a motor is improved, if the width of the bridge 14 is made narrower. However, a narrower bridge 14 tends to generate magnetic saturation and causes electromagnetic noise. A large electromagnetic noise is generated particularly during a high-torque operation when a capacitance of a driving capacitor is large.
The rotor 9 in FIG. 12 shows an example of open slots, having slot apertures 12a instead of the bridges 14 connecting each tooth 13 between the slot portion 12 and the rotor circumference 9a. However, the rotor circumferences 9a have to be cut off or buffed up after die-casting process which is carried out after a core laminating process, because aluminum spouts out of the slot aperture 12a toward the circumference during the aluminum die-casting process.
A conventional single-phase induction motor constructed in the above-mentioned way has following problems.
(1) The rotor 9 having closed slots tends to generate magnetic saturation due to narrow bridges 14 during a high torque operation when a capacitance of a driving capacitor is large, and generates an electromagnetic noise which makes a large noise during operation.
(2) The rotor 9 having open slots needs cutting-off or buffing-up of the rotor circumference 9a after die-casting, because aluminum spouts out of the slot apertures 12a toward the circumference during the aluminum die-casting process.